Selective excitation of a gaseous region

ABSTRACT

Apparatus for the selective excitation of a gaseous region including a chamber enclosing a gas and containing cathode means having a plurality of portions positioned in a first predetermined pattern, and anode means having a plurality of portions positioned in a corresponding pattern. A voltage normally insufficient to cause conduction is impressed between anode and cathode and scan means is provided for causing successive corona discharges which sequentially establish charge carriers in the space between successive portion of the cathode and successive portions of the anode. The charge carriers, in turn, enable heavy conduction between cathode and anode in successive regions of space. The scan means thus addresses the heavy conduction, which can be viewed directly, as in a plasma display device, or used to establish desired modes od oscillation in a laser capable of a plurality of modes of oscillation.

United States Patent [1 1 Beiser et al.

[111 3,774,063 1451 Nov. 20, 1973 SELECTIVE EXCITATION OF A GASEOUS REGION [73] Assignee: Columbia Broadcasting System, Inc.,

New York, NY.

[22] Filed: Sept. 27, 1971 [21] Appl. No.: 184,016

ANODE ELECTRODE (1 of n) (1 of n) COMMON ANODES Primary Examiner-Roy Lake Assistant Examiner-Siegfried H. Grimm Att0rney-Granville M. Brumbauch et al.

[5 7] ABSTRACT Apparatus for the selective excitation of a gaseous region including a chamber enclosing a gas and containing cathode means having a plurality of portions positioned in a first predetermined pattern, and anode means having a plurality of portions positioned in a corresponding pattern. A voltage normally insufficient to cause conduction is impressed between anode and cathode and scan means is provided for causing successive corona discharges which sequentially establish charge carriers in the space between successive portion of the cathode and successive portions of the anode. The charge carriers, in turn, enable heavy conduction between cathode and anode in successive regions of space. The scan means thus addresses the heavy conduction, which can be viewed directly, as in a plasma display device, or used to establish desired modes od oscillation in a laser capable of a plurality of modes of oscillation.

13 Claims, 10 Drawing Figures DEFLECTION MEANS ELECTRON SEC D TRIGGERED Pg DISCHARGE CATHODE WER ELECTRODE SUPPLY -38 (1 of n) PATENTEDNHVZO 1973 3; 774.063 SHEET 1 [IF 4 DEFLECTION MEANS TRIGGER PIN ELECTRON ANODE ELECTRODE ELECTRON GUN SECONDARY TRIGGERED H. v. POWER DISCHARGE CATHODE ANoDE ELECTRODE ARRAY I0 44 I I I I I 1 |4A GASEOUS MEDIUM v, CATHODE Ill] 1 I 1/ J V .l V J V 1 2g ARRAY TRIGGER IN I 9 GLASS ASSEMBLY SCANNING ELscrRoI gy 58 60 62 SUBSTRATE BEAM TRIGGER PINS TRIGGER PINS 54 PERFORATED CATHODE ARRAY PAIENTEUNUVZO ma 3,774,063 SWEET 4 BF 4 MIRROR M IDLER STAGE DRIVER STAGE PRESSURE ELECTRON BEAM ISOLATION CONTROL SECTION BARRIER FIG. 6

cathode F/G. 5A

SELECTIVE EXCITATION OF A GASEOUS REGION BACKGROUND OF THE INVENTION This invention relates to selective excitation of a gaseous region and, more particularly, to novel and highlyeffective selective excitation having particular utility in plasma display devices and in establishing desired modes of oscillation a laser capable of a plurality of modes of oscillation.

An application of Leo Beiser Ser. No. 750,256, filed Aug. 5, 1968, for Laser Systems and Laser Control Systems, now abandoned in favor ofa pending continuation thereof, Ser. No. 99,661, filed Dec. 18, 1970, and assigned to the assignee of the present invention, disclose a laser resonator including nodally-mounted, passive, refractive or reflective image-forming means separate from the lasing medium and conjugate reflective surfaces on which conjugate images are formed. Activation of a desired mode or modes of the laser is achieved by selective excitation. A number of means of activating the desired mode of oscialltion are disclosed in those applications, including the use of a primary or secondary electron flux traversing the gaseous lasing medium in any of a plurality of regions that can be varied in a desired manner.

SUMMARY OF THE INVENTION One object of the present invention is to provide for the establishment of desired modes of oscillation in a laser capable of a plurality of modes of oscillation by means of apparatus capable of operating at higher power than any heretofore available.

Another object of the invention is to provide selective excitation for addressing a heavy gaseous discharge in a plasma display device.

The foregoing and other objects of the invention are achieved by the provision of selective excitation apparatus comprising a chamber for enclosing a gas, cathode means mounted in the chamber and having a plurality of portions positioned in a first predetermined pattern, and anode means mounted in the chamber and having a like plurality of portions positioned in a corresponding pattern, the cathode means and the anode means being in spaced-apart relation to each other. Bias means is provided for impressing a voltage between the anode means and the cathode means normally insufficient to cause conduction therebetween. Scan means is provided for causing successive corona discharges which sequentially establish charge carriers in the space between successive portions of the cathode means and successive portions of the anode means. The

cahrge carriers, in turn, establish heavy conduction be-- tween the cathode means and the anode means in successive regions of space.

The chamber is preferably formed at least in part of a transparent material making the heavy conduction visible from positions outside the chamber.

Since it is isolated within the chamber, the gas employed can be of a normally contaminating type, and, in particular, can be carbon dioxide (CO which is desirable in applications where high laser power generation (such as at l0.6 ,u.) is required.

Preferably, the scan means for establishing the corona discharge comprises an electron gun mounted in spaced-apart relation to the chamber for directing an electron beam at an isolating wall portion of the chamberand deflection means for causing the electron beam to execute a scan over a portion of the isolating wall. Conductive trigger means is provided on the isolating wall having a plurality of first portions arranged in the path of the scan on the outside of the chamber and a plurality of second portions respectively positioned relatively to the plurality of portions of the cathode means in such a manner as to cause successive corona discharges to occur in substantial synchronism with the scan by the electron beam of the plurality of first portions of the conductive means.

The second portions of the conductive means can be either positioned within the chamber in spaced-apart relation to the plurality of portions of the. cathode means, or connected to the plurality of portions of the cathode means.

The apparatus can be viewed directly as a display and is also adapted for use in combination with a laser capable of operating in a plurality of modes of oscialltion and mounted with respect to the chamber in such a manner that different modes of oscillation of the laser are successively established in response to the heavy conduction between the cathode and anode means in successive regions of space. When employed for laser control, more than one such device may be disposed within the resonator to either synchronously or independently enhance the gain region. If synchronously, the controlled gain is enhanced; if independently an X-Y raster scan may, for example, be accomplished.

Extinction means responsive to the heavy conduction is preferably provided for causing sequential cessation of the heavy conduction. The extinction means preferably comprises either an R-C network in circuit with either the anode means or the cathode means for reducing the voltage between successive aligned portions of the anode and cathode means between which the heavy conduction has been initiated sufficiently that the heavy conduction therebetween ceases. Alternatively, extinction may be accomplished by establishing convection between the anode means and the cathode means, whereby the heavy conduction, upon being initiated between successive coacting portions of the anode and cathode means occurs over paths, progressively lengthened sufficiently that the heavy conduction ceases.

BRIEF DESCRIPTION OF THE DRAWING An understanding of additional aspects of the invention can be gained from a consideration of the following detailed description of the preferred embodiments thereof in conjunction with the appended figures of the drawing, wherein:

FIG. 1 is a diagrammatic perspective view of selective excitation apparatus in accordance with the invention;

FIG. 2 is a fragmentary view in sectional elevation of another embodiment of apparatus constructed in accordance with the invention;

FIG. 2A is a view taken generally along the line 2A-2A of FIG. 2 and looking in the direction of the arrows;

FIG. 3 is a fragmentary view in sectional elevation of another embodiment of apparatus constructed in accordance with the invention;

FIG. 3A is a view taken generally along the line 3A-3A of FIG. 3 and looking in the direction of the arrows;

FIG. 4 is a fragmentary view in sectional elevation of another embodiment of apparatus constructed in accordance with the invention;

FIG. 4A is a view taken generally along the line 4A-4A of FIG. 4 and looking in the direction of the arrows;

FIGS. 5 and 5A are schematic circuit diagrams of two embodiments of extinction means in accordance with the invention; and

FIG. 6 is a diagrammatic view of apparatus in accordance with the invention wherein different modes of oscillation of a laser are established in repsonse to triggering by selective excitation apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. I shows selective excitation apparatus 10 including a rectangular housing or chamber 12 formed, for example, of glass-epoxy for enclosing a gas within a space 14. In one embodiment discussed below, opposite ends of the chamber are provided with openings to accept gas input andexhaust flow tubes (not shown). Mounted inside the chamber 12 is cathode means 16 having a plurality of portions 18, 20, 22, 24,...n positioned in a first predetermined pattern and anode means 28 having a like plurality of portions 30, 32, 34, 36,...n positioned in a corresponding predetermined pattern.

Corresponding portions of the cathode means 16 and the anode means 28 are in spaced apart relation to each other. A high voltage power supply 38 is connected by a common conductor 40 to the anodes 30, 32, 34, 36,...n and the cathode portions 18, 20, 22, 24, ...n are connected to ground potential via a common ground lead 42. In this manner, a voltage is impressed between the anode means 28 and the cathode means 16. This voltage is normally insufficient to cause conduction between the anode means 28 and the cathode means 16.

Successive corona discharges, which sequentially establish charge carriers in the space between successive portions l8n of the cathode means 16 and successive portions 30-n of the anode means 28, are caused by scanning means generally indicated at 44. The charge carriers, in turn, enable heavy conduction between the cathode means 16 and the anode means 28 in successive regions of space.

The scanning means 44 comprises an electron gun mounted outside the chamber 12 and operative to direct an electron beam 52 through an evacuated tube 53 toward the chamber 12. The inner end of the tube 53 is joined to the wall 46 of the chamber, at least that portion corresponding to the area of the tube being formed of insulating material. Thus, the wall 46 isolates the gas contained within the chamber 12, which is of a type that would otherwise contaminate components of the electron gun, from the electron source. Deflection means 50 is provided for causing the electron beam 52 to execute a scan over a portion of the wall 46.

Conductive means 54 such as a set of nickel trigger pins is provided on the insulating wall 46 and includes a plurality of first portions 56, 58, 60 and 62...n arranged in the path of the scan of the electron beam 52 on the outside of the chamber 12 and a plurality of second portions 64, 66, 68, 70,...n extending through the wall 46 into the chamber and respectively positioned relatively to the plurality of portions of the cathoce means 18, 20, 22, 24,...n in such a manner as to cause the successive corona discharges to occur in substantial synchronism with the scan of the electron beam across the plurality of first portions 56, 58, 60, 62,...n of the conductive means. Background information relating to corona discharge triggering is found in a book entitled Spark Chambers, O.C. Allkofer, Verlag Karl Thiemig KG. (1969) pp.l8l-202.

FIGS. 2 and 2A and 4 and 4A show arrangements wherein the second portions 54 of the trigger pins are respectively positioned within the chamber 12 in spaced-apart relation to the plurality of portions of the cathode means 16. The trigger pins are embedded in a glass substrate (FIG. 4) or other suitable material, and the electron beam 52 sweeps the pins in any desired pattern. The anode array, which may be discontinuous as shown, or continuous, serves as the electrode at which the discharge terminates.

FIGS. 3 and 3A show an embodiment wherein the conductive means or trigger pins 54 are connected to respective ones of the cathode means 16.

In the embodiments of FIGS. 2 and 3, the chamber 12 is formed of transparent material and includes Brewster angle windows and 70' mounted, for example, parallel to each other and to walls 7 L and 71' of the chamber so that the heavy conduction is visible from outside the chamber. In particular, the Brewster angle windows 70 and 70' are mounted in opposite walls of the chamber making the heavy conduction visible from positions outside the chamber and on opposite sides thereof. In the case of a laser configuration, the windows 70 and 70 may comprise the mirror or mirrors of the laser resonator. I

The gas confined within the chamber is one selected to yield a heavy discharge, a particularly desirable example being CO Such gases may be chemically active or otherwise contaminate thermionic cathodes, but this causes no problem, since the gas is confined within the chamber (or within the chamber 12 plus its associated pump and piping, if the latter are used), and thus does not come into contact with the cathode of the scanning electron gun.

In accordance another aspect of the invention, extinction means is provided for'causing a sequential cessation of the heavy conduction and preparing the region of heavy conduction for the next triggering excitation. By way of illustration, four such extinction means are described below.

The extinction means in one preferred embodiment comprises R-C network 72 shown in FIG. 5 which is connected in circuit with the anode means 28 for reducing the voltage between successive given portions of the anode means and successive given portions of the cathode means between which the heavy conduction has been initiated. This reduction is effective by discharge of a capacitor 74 (previously charged by a d-c source, such as a battery 76) upon initiation of the heavy conduction. Afer such discharge, current from the d-c source 76 recharges the capacitor 74 through a resistor 78, resulting in a voltage drop thereacross and hence at the anode means 28. The parameters of the circuit are selected to cause the voltage drop to be sufficient that the anode-to-cathode conduction is no longer supported.

FIG. 5A shows another embodiment of extinction means comprising an R-C network 80 connected in circuit with the cathode means 16, which is a physically convenient location for it. The cathode 16 is initially at ground potential, so that the capacitor82 is discharged through resistor 84. Upon initiation of heavy conduction, the capacitor 82 becomes charged, causing an increase in the voltage across the resistor 84, and elevating the potential of the cathode 16 above ground. This reduces the anode-to-cathode voltage drop sufficiently that the heavy conduction ceases.

A separate circuit of the type shown in FIG. 5 or FIG. 5A is, of course, employed in association with each of the heavy discharges to be separately extinguished. Active electrical circuits, for example, circuits utilizing transistors and diodes, well known to the art, may be employed to further control the discharge rate.

A third extinction means involves the utilization of convection. In the embodiments of FIGS. 2 and 2A and 3 and 3A, gas flow represented by an arrow A establishes convection between the anode means 28 and the cathode means 16, whereby the heavy conduction, upon being initiated between successive corresponding given portions of the anode means 28 and the cathode means 16, occurs over paths progressively lengthened sufficiently that the heavy conduction between the successive given portions of the anode and cathode means ceases. The convection is established by means of a pump P (FIGS. 2 and 3) which conveys the gas through the gas input and exhaust flow tubes mentioned above.

Interestingly, the convection form of extinction can also serve as a scanning means if the convection is from one trigger point to an adjacent trigger point. In such case, the heavy conduction ceases at the one point and commences at the adjacent point, in response to the convection. The processes of both extinction and elemental scan may also be accomplished with controlled magnetic fields, represented by the arrows labeled B in FIG. 3, which develop forces normal to both the field direction and the plasma discharge axis, in a manner well known to the art of plasma confinement and control.

The apparatus described above can be viewed directly as in a plasma display device, the electron source and scan means 44, 50 serving to address the location of the visible point of light. The scan can be in any desired pattern: for example, a linear scan, a raster scan, or a highly selective scan in which the electron beam jumps from trigger element to trigger element in any desired pattern, with appropriate blanking. The duration of the excitation can correspond closely to the persistence of a conventional television phosphor screen or can be made greater or less, as may be desired.

FIG. 6 shows employment of the selective excitation apparatus 10 in a split confocal transversely degenerate resonator 90 having an end mirror M an end mirror M and a center mirror M An idler stage 92 is provided in one leg, which is preferablyd-C excited to a level below the lasing threshold, thus reducing losses within the resonator, and selectively activated in desired modes by control of the electron beam 52 of the apparatus 10 in the other leg. For example, as the beam 52 sweeps in a direction perpendicular to the plane of FIG. 6, the location of the heavy conduction moves in scan-like fashion in the same direction, and the activated laser modes move likewise. If more than one such control stage is employed, the variable gain region may be enhanced either synchronously to yield a greater gain differential, or independently to provide, for example, two-dimensional scan.

In FIG. 6, 8, is the minimal mode cross section, R is the separation between the mirrors M and M on the one hand and mirror M on the other, and 2a is the mirror'aperture width, which is equal to 8 in this symmetric resonator example.

Keeping in mind that the localized excitation region (in the case of laser control) is confined approximately to the ellipsoidal-shaped intersection volume of the plasma discharge and the most coincident laser mode volume, several methods for isolating and confining the discharge can be employed. For example, a plurality of thin glass plates or slides S, say, seven mils thick, may be placed between adjacent cathode electrodes to provide controlled individual plasma triggering. The plates preferably are of a length to extend to either side of the region containing the electrodes and have a width to extend from the wall supporting the cathode electrodes at least to the level of the tips of the trigger pins, as shown. Alternatively, the glass plates may extend completely across the space between walls 71 and 71' so as to also provide isolation of the individual anode electrodes. As shown in FIG. 2, the thus localized plasma between a cooperating cathode-anode pair is swept into the mode volume and caused to lie along the axis of the laser cavity throughout a major portion of its length by the flowing gas system.

Axial magnetic confinement may be provided with a simple permanent magnet circuit (not shown). A magnetic field as low as approximately 25 gauss produces noticeable confinement.

Insulating ceramic tubing T (FIGS. 2 and 3) can be used to cover the electrode leads and expose varying amounts of the tips of the pins, thereby shielding the conductors from all but the desired ends of the triggering circuit. This simple measure improves plasma stability.

Alternatively, a shielded electrode structure can be employed, as in FIGS. 3 and 3A, in which the trigger and cathode pins are common and the section of the electrode extending into the plasma region is surrounded by a ceramic shield. The pins can be staggered to allow close spacing. This configuration is compact, adjacent pins are isolated, and the laser aperture remains unobstructed.

An important advantage of the invention is that significant power amplifications, as measured from the low power electron beam to the trigger discharge, can be achieved. For example, triggered discharges exhibiting a peak current of 0.5 ampere, corresponding to a peak power of 750 watts, for durations as long as 20 milliseconds, can be achieved. The electron beam power need be only approximately 10 watt; this represents a power gain of 10 to 10 Even without axial magnetic field confinement, the discharge size can be as little as about 1 millimeter in width over a length of about 20 millimeters. The plasma discharge can be swept from trigger pin to trigger pin with the scan of the electron beam at speeds as high as 0.2 milliseconds per sweep.

Gas mixtures of CO zN zl-le in the proportion of 2:4:7 may be employed at a flow rate of liters per minute. Lasing power at 10.6 microns, coincident with the triggering electrical pulses, can be detected.

Thus, there is provided in accordance with the invention novel and highly-effective selective excitation apparatus. The gas within the chamber 12 can be at optimum pressure and of any desired composition for maximum efficiency, while the electron beam 52 operates in a high vacuum, since there is complete pressure isolation between the chamber 12 and the electron source. Many modifications of the representative embodiments described above will now occur to those skilled in the art. For example, it is within the scope of the invention to employ a plurality of electron beams or other scanning means so that a plurlaity of heavy gas discharges can be initiated simultaneously at widely spaced points. Accordingly, the invention includes all of the embodiments thereof within the scope of the appended claims.

We claim:

I. Selective excitation apparatus comprising chamber means for enclosing a gas, cathode means mounted in said chamber means and having a plurality of portions positioned in a first predetermined pattern, anode means mounted in said chamber means and having a plurality of portions positioned in a second predetermined pattern corresponding to said first predetermined pattern, said cathode means and said anode means being in spaced-apart relation to each other, bias means for impressing a voltage between said anode means and said cathode means normally insufficient to cause conduction therebetween, scan means operative to cause corona discharges followed by heavy conduction between successive cooperating portions of said cathode means and said anode means in successive re gions of space, and extinction means responsive. to said heavy conduction for causing a sequential cessation of said heavy conduction.

2. Apparatus according to claim ll wherein said chamber means is formed at least in part of a material which is transparent whereby said heavy conduction is detectable from a position outside said chamber means.

3. Apparatus according to claim 1 wherein at least portions of opposite sides of said chamber means comprise a material which is transparent over a wavelength of interest whereby said heavy conduction is detectable from positions outside said chamber means and on opposite sides thereof.

4. Apparatus according to claim 1 further comprising a like plurality of trigger elements mounted in cooperative relationship with cooperating portions of said cathode means and said anode means, and wherein said scan means further includes means for applying control signals in a predetermined sequence to selected ones of said trigger elements for initiating said successive corona discharges in substantial synchronism with said control signals.

5. Apparatus according to claim 1 wherein said extinction means comprises circuit means connected in series with each of said cooperating portions of said cathode means and said anode means and operative to terminate said heavy conduction between selected ones of said cooperating portions by increasing the electrical impedance of all but the selected cooperating portions. 6. Apparatus according to claim 1 wherein said chamber means is formed at least in part by an isolating wall and wherein said scan means comprises electron gun means mounted in spaced-apart relation to said chamber means for directing an electron beam at said isolating wall, deflection means for causing said electron beam to execute a scan over a portion of said isolating wall, and conductive means projecting through said isolating wall having a plurality of first portions arranged in the path of said scan on the outside of said chamber means and a plurality of second'portions respectively positioned relatively to said plurality of portions of said cathode means in such'a manner as to cause said successive corona discharges to occur in substantial synchronism with said scan by said electron beam of said plurality of first portions of said conductive means.

'7. Apparatus according to claim 6 wherein said second portion of said conductive means are positioned within said chamber means in spaced-apart relation to respective ones of said plurality of portions of said cathode means.

8. Apparatus according to claim 6 wherein said conductive means are connected to respective ones of said plurality of portions of said cathode means.

9. Apparatus according to claim 1 further comprising laser means capable of a plurality of modes of oscillation and mounted with respect to said chamber means in such a manner that different modes of oscillation of said laser means are successively established in response to said heavy conduction between said cathode means and said anode means in successive regions of space.

10. Apparatus according to claim 1 wherein said extinction means comprises electrical network means in circuit with said anode means for reducing the voltage between successive given cooperating portions of said anode means and said cathode means between which said heavy conduction has been initiated sufficiently that said heavy conduction between said successive given cooperating portions of said anode means and said cathode means ceases.

11. Apparatus according to claim 1 wherein said extinction means comprises electrical network means in circuit with said cathode means for reducing the voltage between successive given cooperating portions of said anode means and said cathode means between which said heavy conduction has been initiated sufficiently that said heavy conduction between said successive given cooperating portions of said anode means and said cathode means ceases.

12. Apparatus according to claim 1 wherein said extinction means comprises means for establishing convection between said anode means and said cathode means, whereby said heavy conduction, upon being initiated between successive given portions of said anode means and "successive given portions of said cathode means, occurs over paths progressively lengthened sufficiently that said heavy conduction between said successive given portions of said anode means and said successive given portions of said cathode means ceases.

l3. Selective excitation apparatus comprising chamber means for enclosing a gas, cathode means mounted in said chamber means and having a plurality of portions positioned in a first predetermined pattern, anode means mounted in said chamber means and having a plurality of portions positioned in a second predetermined pattern corresponding to said first predetermined pattern, said cathode means and said anode -means being in spaced-apart relation to each other,

bias means for impressing a voltage between said anode means and said cathode means normally insufficient to cause conduction therebetween, and scan means for initiating a corona discharge followed by heavy conduction in a given region between said anode means and said cathode means and for establishing convection from said given region to an adjacent region between said anode means and said cathode means in which said heavy conduction can be supported, whereby said heavy conduction between said cathode means and said anode means occurs in scan-like fashion in successive regions of space, said convection also causing a sequential cessation of said heavy conduction.

i #k =l l 

1. Selective excitation apparatus comprising chamber means for enclosing a gas, cathode means mounted in said chamber means and having a plurality of portions positioned in a first predetermined pattern, anode means mounted in said chamber means and having a plurality of portions positioned in a second predetermined pattern corresponding to said first predetermined pattern, said cathode means and said anode means being in spacedapart relation to each other, bias means for impressing a voltage between said anode means and said cathode means normally insufficient to cause conduction therebetween, scan means operative to cause corona discharges followed by heavy conduction between successive cooperating portions of said cathode means and said anode means in successive regions of space, and extinction means responsive to said heavy conduction for causing a sequential cessation of said heavy conduction.
 2. Apparatus according to claim 1 wherein said chamber means is formed at least in part of a material which is transparent whereby said heavy conduction is detectable from a position outside said chamber means.
 3. Apparatus according to claim 1 wherein at least portions of opposite sides of said chamber means comprise a material which is transparent over a wavelength of interest whereby said heavy conduction is detectable from positions outside said chamber means and on opposite sides thereof.
 4. Apparatus according to claim 1 furthEr comprising a like plurality of trigger elements mounted in cooperative relationship with cooperating portions of said cathode means and said anode means, and wherein said scan means further includes means for applying control signals in a predetermined sequence to selected ones of said trigger elements for initiating said successive corona discharges in substantial synchronism with said control signals.
 5. Apparatus according to claim 1 wherein said extinction means comprises circuit means connected in series with each of said cooperating portions of said cathode means and said anode means and operative to terminate said heavy conduction between selected ones of said cooperating portions by increasing the electrical impedance of all but the selected cooperating portions.
 6. Apparatus according to claim 1 wherein said chamber means is formed at least in part by an isolating wall and wherein said scan means comprises electron gun means mounted in spaced-apart relation to said chamber means for directing an electron beam at said isolating wall, deflection means for causing said electron beam to execute a scan over a portion of said isolating wall, and conductive means projecting through said isolating wall having a plurality of first portions arranged in the path of said scan on the outside of said chamber means and a plurality of second portions respectively positioned relatively to said plurality of portions of said cathode means in such a manner as to cause said successive corona discharges to occur in substantial synchronism with said scan by said electron beam of said plurality of first portions of said conductive means.
 7. Apparatus according to claim 6 wherein said second portion of said conductive means are positioned within said chamber means in spaced-apart relation to respective ones of said plurality of portions of said cathode means.
 8. Apparatus according to claim 6 wherein said conductive means are connected to respective ones of said plurality of portions of said cathode means.
 9. Apparatus according to claim 1 further comprising laser means capable of a plurality of modes of oscillation and mounted with respect to said chamber means in such a manner that different modes of oscillation of said laser means are successively established in response to said heavy conduction between said cathode means and said anode means in successive regions of space.
 10. Apparatus according to claim 1 wherein said extinction means comprises electrical network means in circuit with said anode means for reducing the voltage between successive given cooperating portions of said anode means and said cathode means between which said heavy conduction has been initiated sufficiently that said heavy conduction between said successive given cooperating portions of said anode means and said cathode means ceases.
 11. Apparatus according to claim 1 wherein said extinction means comprises electrical network means in circuit with said cathode means for reducing the voltage between successive given cooperating portions of said anode means and said cathode means between which said heavy conduction has been initiated sufficiently that said heavy conduction between said successive given cooperating portions of said anode means and said cathode means ceases.
 12. Apparatus according to claim 1 wherein said extinction means comprises means for establishing convection between said anode means and said cathode means, whereby said heavy conduction, upon being initiated between successive given portions of said anode means and successive given portions of said cathode means, occurs over paths progressively lengthened sufficiently that said heavy conduction between said successive given portions of said anode means and said successive given portions of said cathode means ceases.
 13. Selective excitation apparatus comprising chamber means for enclosing a gas, cathode means mounted in said chamber means and having a plurality of portions positioned in a first predetermined Pattern, anode means mounted in said chamber means and having a plurality of portions positioned in a second predetermined pattern corresponding to said first predetermined pattern, said cathode means and said anode means being in spaced-apart relation to each other, bias means for impressing a voltage between said anode means and said cathode means normally insufficient to cause conduction therebetween, and scan means for initiating a corona discharge followed by heavy conduction in a given region between said anode means and said cathode means and for establishing convection from said given region to an adjacent region between said anode means and said cathode means in which said heavy conduction can be supported, whereby said heavy conduction between said cathode means and said anode means occurs in scan-like fashion in successive regions of space, said convection also causing a sequential cessation of said heavy conduction. 